Never write off a rank outsider. A female mammal that carries a “male” chromosome should struggle to reproduce, but not if that animal is an African pygmy mouse. For females of this species, a male chromosome spells reproductive supremacy.

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The mouse’s most remarkable feature is hidden within its cells. In the 1960s, geneticists found three sex chromosomes floating in its gene pool, rather than the two most mammals carry. As well as the normal X and Y sex chromosomes, there is a modified X called X* (Experientia, doi.org/bvsx8x).

In 2010, Frédéric Veyrunes of the Institute of Evolutionary Sciences at the University of Montpellier in France and his colleagues realised the X* chromosome was special. It has mysterious “super-feminising” properties that block the masculinising effect of the Y chromosome (Proceedings of the Royal Society B, doi.org/c8s258). This means there are three kinds of female African pygmy mouse. Some are the standard XX, some an unusual XX* and around 75 per cent are X*Y. Males are all XY.

Muddled sex chromosomes are common in fish and amphibians, but not in mammals. For nearly 150 million years, most female mammals have been XX and males have been XY. Of 5400 mammal species, Y-chromosome females are common in just two kinds of lemming, a few South American field mice, and the African pygmy mouse.

Tomboys rule OK

Even among these oddities, the African pygmy mouse stands out, because females with a Y chromosome do better than those without. Veyrunes and colleagues have found that, compared with other females, those with a Y chromosome were significantly more likely to have at least one litter during a six-month spell in contact with a male. Y-chromosome females also had larger litters, averaging almost one extra baby.

That is remarkable because one-quarter of a Y-chromosome female’s offspring will die thanks to inheriting two Y chromosomes, one from each parent, leaving the offspring without vital genes found only on the X chromosome. That should prevent Y-chromosome females from even matching other females’ reproductive rate, let alone out-competing them.

“We didn’t expect X*Y females to ‘outperform’ XX and XX* females,” says team member Paul Saunders. “The African pygmy mouse is a close relative of the house mouse, in which Y-chromosome females are usually sterile.”

Their reproductive success explains why Y-chromosome females are so common among African pygmy mice. However, they can never completely replace their non-Y sisters, because an X*Y female and XY male will always make some XX* female offspring as well as more X*Y females.

What’s so great?

But why does having a Y chromosome boost a female’s reproductive performance? One idea is that the super-feminising X* chromosome the Y-chromosome females carry is really responsible. But it cannot be, says Saunders, as XX* females are no more reproductively successful than XX females.

Part of the answer might simply be that males and Y-chromosome females are more likely to interact, and ultimately breed, because the females behave in a more “masculine” way. “Behavioural differences between XX and Y-chromosome females have been investigated in lab mice,” says Saunders. “There were differences in aggressiveness and social interaction with intruders.”